The present invention relates to methods for separating component gases from a feed gas stream and more particularly, to a process for removing xenon and radioactive krypton from the off-gas streams of nuclear processes.
Nuclear power generation and nuclear fuels reprocessing generate several types of radioactive waste, including an off-gas stream which contains various concentrations of radioactive Kr-85. These concentrations are generally in excess of levels which can be safely released to atmosphere due to high levels of radioactivity and consequently, processes have been devised in attempts to remove radioactive krypton from such gas streams. The removed krypton is held in suitable storage vessels until radioactive decay is sufficient to enable subsequent release or use of such krypton.
Previously proposed processes typically include the removal from nuclear process off-gas streams of certain constituents in a "warm" end and other constituents in a cryogenic portion of the process. For example, oxygen and oxides of nitrogen may be removed by catalytic combustion and the resulting water may be dried from the main stream gas in appropriate devices in the warm end of the system. Usually, carbon dioxide is adsorbed from the resulting feed gas stream in suitable adsorber devices in the warm end of the system while xenon and krypton are separated from the feed gas stream in cryogenic distillation towers or the like. Previously proposed techniques for decontaminating nuclear process off-gas streams have suffered from several drawbacks. Generally, such processes do not assure virtually complete removal of radioactive components from the off-gas stream and these processes have been ineffective in reducing to practical levels the volumes of gases from which components such as radioactive krypton are removed. This latter deficiency of previously proposed processes results in excessive capital costs because of the relatively large volumes of gas involved and the manner in which such processes have attempted to decontaminate the same. For example, it has been proposed in U.S. Pat. No. 4,080,429 to separate krypton from the off-gas stream of a nuclear process. However, in this separation process, helium is specifically relied upon as a carrier gas and consequently, the overall volume of gases treated is relatively large. This, in turn, results in concomitant increased capital costs of apparatus necessary to treat a given volume of contaminated gas. Other proposed prior art processes include the method described in U.S. Pat. No. 3,944,646 in which it is stated that krypton and xenon are separated from one another in a single column. Although this type of separation may operate in the manner described, the process illustrated in this reference relies upon a total reflux of liquid nitrogen and thus results in a process relatively expensive to operate in that large quantities of `utility` or commercially pure nitrogen liquid are required.
In another process described in the prior art, namely U.S. Pat. No. 4,012,490, which is assigned to the assignee of the present invention, it is proposed to separate carbon dioxide and xenon in a single adsorber stage and recover this combined material for storage. It has been found, however, that due to the solubilities of the constituents involved, the separation of a combined CO.sub.2 -xenon product is not entirely feasible and may result in the unavoidable inclusion of krypton with such combined product. A similar process is illustrated in U.K. Patent Specification No. 1,525,686.
Consequently, a review of prior art processes indicates a clear need for a technique for decontaminating nuclear process off-gas streams in a manner which is thorough, safe and yet does not incur excessive capital or equipment costs and is both reliable and efficient to operate on a virtually continuous basis.